1. Field of the Invention
This invention relates to methods ad apparatus for making clad sheets, which have high bonding strength, by low reduction cold rolling in vacuum without heating.
2. Description of the Prior Art
There have been many manufacturing processes developed for the cladding of a variety of metals with other dissimilar metal, such as (1) hot rolling, (2) cold rolling, (3) explosion bonding, (4) a combination of processes mentioned using insert metal, (5) brazing, (6) welding, and (7) using organic adhesive material. Of these processes, (1), (2), and (3) are continuous processes for cladding of metal sheet with other metal sheet, which achieve tight metal bonding.
In the case of hot rolling to clad steel with a dissimilar metal, it is necessary to heat above 800.degree. C., therefore it is impossible to clad with a metal having a melting point lower than 800.degree. C. In many cases of the combination of dissimilar metals, thick brittle intermetallic alloys or compounds, which result in poor formability of the clad sheet product, are created at high rolling temperature. The residual oxide films in the bonding interface occasionally introduce bonding defects and the oxide films formed during heating spoil the surface appearance.
In the case of cold rolling, more than 50% reduction by one pass is needed to obtain the tight bonding in a heterogeneous metal combination. Therefore annealing after rolling is necessary to recover the ductility of the product, but formability is deteriorated by intermetallic compounds created during annealing.
To solve these problems, low reduction cold rolling, in which the metal surfaces to be bonded are previously cleaned to improve the weak bonding strength, is tried. For example, as shown in Japanese patent application laid open No. Sho. 59-92186, the product of the oxygen fraction of the atmosphere (weak oxidizing or non oxidizing atmosphere), in which the metals to be bonded are cleaned by shot blasting or wire brushing and rolled, and the time needed from cleaning to rolling, must be controlled within a certain range. But by this method, it is difficult to prevent the absorption of the gas molecules or atoms different than oxygen, such as hydrocarbon, on brushed and cleaned metal surfaces.
Japanese patent application laid open No. Sho. 61-286078, discloses metal surfaces that are cleaned by ion beam milling before cold rolling. But the ion beam milling has some problems for application in an industrial cladding system: (1) radiation diameter is too small; (2) etching rate is insufficient; (3) efficiency of etching tends to be low due to the charging by the radiation of the ion, because many of the oxide films on the metal surface are an insulator; (4) a 50% reduction for cladding and diffusion heat treatment is required; and (5) an ion beam gun costs dearly. Therefore this process is unsuitable to the considerably high speed cladding system for wide clad sheet.
A method of low pressure bonding at room temperature, which is not a method of making a metal clad sheet, was developed. In this method, the metal surfaces to be bonded are cleaned by ion beam milling and then pressure welded in ultra high vacuum at room temperature. (Japanese patent application laid open No. Sho. 54-124853). It is not suitable for an industrial system because it is essential that the pressure welding is carried out in an ultra high vacuum (usually below 10.sup.-8 Torr) to avoid contamination of cleaned surfaces.